The present invention relates generally to the field of applying coatings to optical fibers during the drawing thereof. More particularly, the invention relates to a method of coating optical fibers in such a manner as to prevent twisting of such fibers and/or to facilitate the proper alignment of such fibers.
It has been known that coupling occurs between two closely spaced cores in a multiple core device. The coupling efficiency increases with decreasing core separation and, in the case of single-mode cores, with decreasing core diameter. Fiber optic couplers employed in systems such as fiber optic gyroscopes must be capable of preserving the polarization of optical signals propagated therethrough. Such couplers are preferably made from single-mode fibers which are capable of preserving the polarization of signals along two different, usually orthogonal axes. Polarization retaining single-mode (PRSM) fiber optic couplers are disclosed in U.S. Pat. Nos. 4,589,725 and 4,755,021 issued to R. B. Dyott and U.S. Pat. No. 4,753,497 issued to Y. Fujii et al. The couplers taught in these patents comprise PRSM fibers of the type wherein the core is surrounded by an elliptically-shaped cladding layer which imparts a stress to the core.
U.S. Pat. No. 4,755,021 teaches that a PRSM fiber having two intersecting flat surfaces can be drawn from a similarly shaped preform. The preform can be made by first forming a cylindrical preform with an elliptical core and cladding located in the center thereof and then grinding two adjacent sides of the preform to form a cross-section having one flat surface parallel to the major axis of the elliptical core and another flat surface parallel to the minor axis of the elliptical core. Two or more of the fibers produced from such a preform can be etched along those portions of their lengths where it is desired to couple the fields of the two fibers. The etched fibers are fed, with their flat sides facing each other, through a glass tube which is then heated until the central region thereof collapses onto the fibers. The etching and the subsequent handling of the etched fibers is a tedious process. Moreover, the cross-sectional shape of these fibers makes it difficult to connect them to conventional single-mode fibers having axial cores.
In U.S. Pat. No. 4,753,497 a circular PRSM fiber is bonded in a channel with the major axis of the elliptical cladding either perpendicular to or parallel to the channel surface. The substrate in which the channel is located and the PRSM fiber embedded therein are then abraided away to a depth sufficient to permit light traveling in the core to escape from the fiber. To form a coupler, two similarly formed substrates are combined with index matching fluid interposed therebetween. Connection of the coupler fibers to the other system fibers is facilitated by the use of circular PRSM fibers. However, to ensure proper functioning fo such a coupler, the technician forming the coupler must know the orientation of the major axes of the PRSM fibers so that they can be positioned in suitable alignment. It is sometimes extremely difficult to determine the relative orientation of a circular PRSM fiber from a transverse observation thereof.